外文原文.pdf

Isothermal sheet formability of magnesium alloy AZ31 and AZ61 Shyong Leea Yung Hung Chena Jian Yih Wangb aDepartment of Mechanical Engineering National Central University Chung li Taiwan ROC bChung Shan Institute of Science and Technology Lung tan Taiwan ROC Received 18 February 2001 Abstract There have been reports on the forming of magnesium alloy sheet in industry but this paper is probably the fi rst formal paper for studying the sheet formability of AZ31 and AZ61 at various elevated temperatures The results indicate that it is feasible to form products from extruded sheets of 0 5 1 3 1 7 and 2 mm thickness Presently the forming of a sheet of 0 5 mm thickness is considered to be a technical achievement by industry There were two kinds of tooling employed punch and punchless The punchless die setting used pressurized gas to press the sheet into a female die cavity This technique applied to Mg alloy is unprecedented and shows potential for industrial utilization As the stretch ability was demonstrated in gas forming punch die pressing should be achievable numerous punching tests being performed to confi rm this 2002 Elsevier Science B V All rights reserved Keywords AZ31 Punch die pressing Isothermal sheet forming 1 Introduction Magnesium alloy is the lightest metal that can be employed for structural use In the past the demand for this alloy as a structural material was not high because of its less availability commercially as well as limited manufac turing methods In recent years the die casting of magne sium alloy has been prevailing in the making of parts in the automotive industry 1 2 and such items as the covers of notebook computers as well as cellular phones However this process is not ideal in making thin walled magnesium structures because an excessive amount of waste material canresult Apotentialsolutionwouldbetoresorttothesheet formingprocess Itiscommonlyrecognizedthat magnesium possesses poor formability at room temperature because of its hexagonal close packed structure 3 4 Fortunately the workability of Mg alloy can be effectively improved by increasing the working temperature e g increasing above 300 8C 2 In this paper the sheet formability of AZ31 and AZ61 at various elevated temperatures is studied to assess the feasibility of forming products from extruded sheets There are two kinds of tooling employed punch and punch less as described in Fig 1 The punchless die setting uses pressurized gas to press the sheet into a female die cavity This method has the advantage of eliminating friction between the workpiece and the punch tool so that the material s stretch ability can be more genuinely exhibited The strain distribution on various locations of the formed product will be studied Further the material fl ow path will be traced and constructed On the other hand the punch die method involves much less stretching effect but also an uneven load distribution so that its failure mode may be quite different from that in the gas forming process 2 Materials and experimental procedure The alloys employed in the sheet forming work are AZ31 andAZ61 inwhichthemagnesium alloys contains 3and6 of aluminum respectively as indicated in the fi rst numerical digit in the designation the last digit represents the zinc content which is 1 in the above cases The material for sheetformingworkwasobtainedbyextrudingabilletof8 in 203 mm diameter 30 in 762 mm length through a die with 0 5 1 3 1 7 and 2 mm openings at 250 8C for AZ31 and280 8C for AZ61 The basic tool for the experiment is a press machine equipped with a furnace offering desired isothermal conditions For thegasforming punchless work only one die is needed which is in rectangular shape of 40 mm width and 120 mm length The depth of the die is 20 mm butcouldbeadjustedto8 12and16 mmbyinserting Journal of Materials Processing Technology 124 2002 19 24 Corresponding author E mail address shyong cc ncu edu tw S Lee 0924 0136 02 see front matter 2002 Elsevier Science B V All rights reserved PII S0924 0136 02 00038 9 dummy blocks The pre formed fl at sheet was positioned on the die the cover plate with a peripheral rail was placed on to clamp the sheet and then the chamber was sealed to enable pressurized gas to mold the sheet towards the contour of the die The input gas pressure needed to be adjusted constantlyinaccordancewiththevaryingsheetconfi guration during the whole forming process Some of the sheets were marked with a grid so that local strain state could be determined by measuring the deformation of the grid For punch die pressing the rectangular shape sheet used the same die as that for gas forming but the die setting had a 2 mm clearance between the punch and the die A circular shape pressing was also performed where the die diameter was 20 mm and there was a 2 mm clearance 3 Results and discussion 3 1 Gas forming of 1 7 mm thick AZ31 rectangular shape 3 1 1 Formability as a function of the gas pressurization rate Several specimens were formed by the gas pressing tech nique to study the formability of the sheet at various com binations of forming depth and temperatures as well as pressure time p t input Two pieces were formed success fully at 410 8C with 8 mm depth following the p t profi les depicted in Fig 2 For this shallow forming only 90 s were needed utilizing a higher p t profi le as compared with other deepercases Furtherformingto12 mmdepthwas performed at the same temperature with the p t profi le shown in Fig 3 This depth formed at a lower temperature 310 8C was also Fig 1 Schematic diagram of the tools for isothermal sheet forming a gas forming with a rectangular die b rectangular punch and die c circular punch and die Fig 2 Pressure time profile leading to the successful forming of a rectangular shaped box of 8 mm depth at 410 8C 20S Lee et al Journal of Materials Processing Technology 124 2002 19 24 completed successfully but it took a higher pressure and a longer time because the material had a larger fl ow stress For the 16 mm case two blow formings were done with one success and one failure due to the different p t inputs employed as depicted in Fig 4 The full depth 20 mm was tried with a p t profi le Fig 5 that was even higher than that for the two 16 mm cases so it was doomed to fail The above two unsuccessful specimens were photographed and are shown in Fig 6 It is seen that failure started at the middle of the long side on the die entrance For all these formingjobs thegaspressureincreasedasthetimeincreased which should be needed for maintaining the fl ow stress in the material at a constant level Considering the forming sheet as a part of spherical shell surface having an instantaneous confi guration and thickness as a function of time then using the equation for calculating the fl ow stress i e s pr 2t where p is the gas pressure r the curvature radius and t the thickness can partially justify the above experimental pres surization rate An advanced stress analysis with accurate modeling may suggest a more ideal p t curve 3 1 2 Strain distribution and the material s flow locus Among the seven gas formed pieces some were marked with grids in order to measure the strain distribution Origin ally the grid was an array of identical circles of 2 5 mm diameter printed on the sheet surface prior to the forming work It can be seen from the deformed grids that the maximum tensile strains are located at the middle of the long side on the upper curved spot Fig 7 Failure would start at this position if the sheet suffered an unfavorable pressuriza tion rate and temperature This measured strains are the fi rst experimentaldisclosureindicatingthatthecommonly Fig 3 Pressure time profile leading to the successful forming of a rectangular shaped box of 12 mm depth at 410 and 310 8C Fig 4 Two different p t inputs for 16 mm depth gas forming at 410 8C resulted in one success and one failure Fig 5 Pressure time profile for the attempt to form a rectangular shaped box of 20 mm depth at 410 8C Fig 6 Failed gas forming of a 1 7 mm thick sheet for the cases of 16 and 20 mm depths S Lee et al Journal of Materials Processing Technology 124 2002 19 2421 Fig 7 Enlarged view of the deformed grids at various locations in the forming of a part to upper 12 mm depth measuring the grids on the concave upper side lower 16 mm depth the grids being on the convex side lower e1denotes the greatest local surface strain whilst e2is that in the perpendicular direction Fig 8 Flow paths of the material under peripheral sealing in the gas forming process assumed plane strain state in the middle of the long side for deriving a p t curve may not be absolutely correct It was noteworthy that the material under the peripheral sealing rail was not fi rmly held but actually had the tendency to slide as shown in Fig 8 At high temperatures the peripheral rail on the cover plate indented the softened sheet and created a groove under pressing load The outer and inner boundaries of the groove were originally parallel however some parts of the inner boundary was displaced inwards indicating that the material under the peripheral sealing rail was still being stretched evenunder the clampingload This mechanismmay be important in achieving successful gas forming 3 2 The gas forming of 0 5 mm thick AZ31 rectangular shape After completion of the gas forming with sheets of 1 7 mm thickness a more challenging task was that for 0 5 mm thickness where success would be considered a technical achievement from the industrial point of view Three pieces were tried with cavity depths of 12 16 and 20 mm The working temperature was chosen to be 330 8C based on the previous feasibility with 310 and 410 8C The initial pressurization rate was set to be 5 kgf cm2 490 kN m2 considering that the thickness is to be decreased by more than two thirds relative to that for the previously worked cases Plots showing the pressurization rate employed for the forming work are provided Fig 9 For the 12 and 16 mm cases the gas forming work was successful only that much more time was consumed com pared to the previous counterparts For the 20 mm case the forming was not successful and failure started from the position under the peripheral rail which is thinner in the beginning because of the indentation of the material due to clamping for sealing the pressurized gas This failure mode is not the same as its 1 7 mm thick counterpart in which failure occurred at the die entrance It can be proposed that the stress is much higher at the die entrance when bending thicker sheet 3 3 The rectangular punch die forming of 1 3 mm thick AZ31 For the punch die press work to 16 mm depth on AZ31 of 1 3 mm thickness in this category nine pieces were tested at room temperature at 329 8C fi ve pieces and at 435 8C three pieces The room temperature work was not a success as was expected At high temperature 435 8C all of the three specimens were formed successfully At intermediate temperature only one out of the fi ve working pieces was formed successfully This indicates that other parameters such as lubrication punch speed and clearance can exhibit an infl uence when the temperature factor is not dominating The failures in this category occurred mostly at the corners Fig 10 where the punch exerted a concentrat ing pulling force to drag the sheet down this is just what the gas forming method can avoid 3 4 The rectangular punch die forming of 2 mm thick AZ61 It is commonly stated that AZ61 is less formable than AZ31 because of its greater aluminum content in the Mg alloy Two pieces of 2 mm thickness were punch pressed to 16 mm deep at 295 8C one was successful and one failed The working temperature was relatively lower than that which was expected to be needed The results strengthen the observation above that there are some infl uential factors other than temperature Fig 9 Pressure time profiles for the forming of rectangular shaped boxes of depth a 12 mm b 16 mm and c 20 mm at 330 8C from 0 5 mm thick sheets S Lee et al Journal of Materials Processing Technology 124 2002 19 2423 3 5 The circular punch die forming of 2 mm thick AZ61 The circular punching of 16 mm depth of up to 18 pieces was performed The yield as a function of temperature is listed in the following That the success and failure results are dictated by the temperature seems quite clear However there is a transition zone in which the temperature is not the only decisivefactor Lubrication the material s pre condition the punching speed and the punch die clearance etc may be additional affecting factors 4 Conclusions Magnesium alloys AZ31 and AZ61 in sheet form can be formed at elevated temperatures by pressurized gas as well as by punch die methods Temperature is the main factor in determining whether the forming can be successful How ever there are still some secondary infl uential factors